1. Background of the Invention
The present invention relates to an audio signal processing apparatus and an audio signal reproducing apparatus for use with for example a DVD reproducing apparatus.
2. Description of the Related Art
Most of sound tracks of movies performed in movie theaters are of surround type. In the latest surround system, a discrete multi-channel system of which signals are recorded in a digital format rather than a matrix format has been used. In such a system, a sound can be freely moved in forward, backward, leftward, rightward, and peripheral directions so as to create an impressive sound field with a presence effect.
In a large-storage disc-shaped record medium for use with a multimedia application such as a DVD, a record format has been standardized for satisfying demands of high picture quality and high sound quality. A DVD can record surround audio signals of digital 5.1-channels. FIGS. 3A and 3B show the comparison of channel structures and sound fields of the digital 5.1-channel system and a conventional analog four-channel system.
FIG. 3A shows the channel structure and sound field of the analog four-channel system. The analog four-channel system is composed of four channels that are L (front left), C (front center), R (front right), and S (surround) channels. FIG. 3B shows the channel structure and sound field of the digital 5.1-channel system. The digital 5.1-channel system is composed of five channels that are L (front left), C (front center), R (front right), Ls (rear left), and Rs (rear right) channels and 0.1 channel of LFE (Low Frequency Effect) channel. The LFE channel is used for a super woofer to reproduce an ultra low frequency component. The information capacity of the LFE channel is as small as {fraction (1/10)} times the information capacity of each of other channels. Thus, the LFE channel is referred to as 0.1 channel.
The DVD reproducing apparatus is structured corresponding to the digital 5.1-channel system. Thus, the DVD reproducing apparatus can reproduce audio signals of 5.1 channels. In addition, the DVD reproducing apparatus has an audio signal processing apparatus that performs a down-mixing process corresponding to the conventional two-channel stereo audio system. When the audio signal processing apparatus performs the down-mixing process, signal levels of digital signals are attenuated so as to prevent excessive audio signals from being generated. The attenuated digital signals are calculated and then converted into analog signals. The analog signals are amplified again.
However, in a method for attenuating digital signals, if the LSB (Least Significant Bit) side is insufficient against the number of quantized original signals, when the signals are attenuated, information of the original signals is lost. Thus, even if the signal levels of the resultant signals are restored to the signal levels of the original signals, lost information on the LSB side is not restored. In other words, a nuance and so forth of music are lost. In addition, since digital signals are attenuated, analog signals are amplified. Thus, amplification factors of analog amplifiers should be raised. Consequently, since noise levels on the analog stage increase, noise of reproduced signals becomes conspicuous. In this case, unless the amplification factors of the analog amplifiers are raised, the users will sense a decrease in signal levels.
Therefore, an object of the present invention is to provide an audio signal processing apparatus and an audio signal reproducing apparatus that perform a down-mixing process without deterioration of sound quality.
To solve the above-described problem, the invention of claim 1 is an audio signal processing apparatus for calculating audio signals of a plurality of channels and forming a combined audio signal, comprising a calculating means for calculating digital audio signals of a plurality of channels, a detecting means for detecting levels of the digital audio signals of the plurality of channels, and a means for predicting whether or not an occurrence of an overflow of the combined audio signal will take place corresponding to an output of a calculating process of the calculating means and an output of the detecting means and for attenuating a digital audio signal of at least one channel of the plurality of channels when the occurrence of the overflow has been predicted.
The invention of claim 2 is an audio signal processing apparatus for calculating audio signals of a plurality of channels and forming a combined audio signal, comprising a calculating means for calculating digital audio signals of a plurality of channels, a detecting means for detecting levels of the digital audio signals of the plurality of channels, and a means for predicting whether or not an occurrence of an overflow of the combined audio signal will take place corresponding to an output of a calculating process of the calculating means and an output of the detecting means and for varying the phase of a digital audio signal of at least one channel of the plurality of channels when the occurrence of the overflow has been predicted.
An audio signal reproducing apparatus, comprising a reproducing means for reproducing digital audio signals of a plurality of channels from a record medium, and a signal processing means for processing reproduced digital audio signals, wherein the signal processing means has a calculating means for calculating digital audio signals of a plurality of channels, a detecting means for detecting levels of the digital audio signals of the plurality of channels, and a means for predicting whether or not an occurrence of an overflow of the combined audio signal will take place corresponding to an output of a calculating process of the calculating means and an output of the detecting means and for attenuating a digital audio signal of at least one channel of the plurality of channels when the occurrence of the overflow has been predicted.
According to the present invention, when signals are down-mixed by a digital calculating process, if an occurrence of an overflow is predicted in a combined signal, an attenuating process or a phase varying process is performed. Since signals are not always attenuated, information on low order bit side can be prevented from being lost. In addition, since analog amplifiers do not need to compensate attenuated signal levels, noise can be prevented from increasing.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.